Long Ago

When we gaze into a starry sky, we’re looking into the past. But just how deep into the history of the universe can we see? A reader’s question about peering into eternity !

Stephen Powelson in Virginia, asks, “Is there a limit to how far back we can peer in the universe’s history?”

We see the stars as they were when light left them — tens, hundreds or thousands of years ago. Telescopes let us look farther away — deeper into the past. And light from distant galaxies has traveled for billions of years to reach us. At present, it’s thought we can see back almost to the Big Bang — the colossal explosion astronomers believe marked the birth of our universe. But is it possible to see all the way back to the Big Bang — to the moment time began?

Almost . . . but not quite. Astronomers now think the Big Bang took place 13.7 billion years ago. Three hundred thousand years later, the universe would still have been so hot that atoms — basic building blocks of matter — couldn’t form. In this hot early universe, particles smaller than atoms are thought to have trapped light. After 300,000 years, atoms did form, and they trapped light, too — but less efficiently. Today, the farthest back we see is an ancient glow — the so-called “surface of last scattering” or “cosmic background radiation.”

Someday scientists might figure out how to see something from farther back in time than 300,000 years after the Big Bang — but they probably won’t be using light.

First off, two factoids: Just in case you were wondering, scientists may be able to use gravity waves or possibly neutrinos to look deeper in time — but no method has been developed. And, on a different topic, in order to form atoms, the universe had to cool down to about 3000K — about the temperature of the surface of a red giant star. Still pretty toasty!

Stephen’s full question was a bit more complicated than the shortened question we answered in the script. I tried to get at the spirit of his question, but realized that there was no way I could hit on everything aspect. I’ll try to do a little more here.

Here’s the full question from Stephen: “Is there a theoretical limit as to how far back we can peer in the universe’s history? One might think that information carried by radiation (light or other) generated very early in the universe’s life would have already passed us by (since the early universe was small) and would no longer be available to us.”

To begin at the beginning: Many scientists believe that the Big Bang was followed by a very fast period of “inflation” (possibly less than 10^-30 of a second). During “inflation” the universe grew exponentially and much more quickly than the speed of light — from something infinitesimally small to something that might be endless. Since then, the space that contains the universe has been expanding. **

Space appears to be larger than the area we can observe. In other words, if the universe has an edge, we are not near it and we cannot see it. So although it is true that early light from nearby stars, galaxies, and particle-filled spaces has indeed “passed us by”, light from more distant locations can still reach us. And the more distant the region, the farther back in time we get to see. That’s why we can see the surface of last scattering, way back at 300,000 years (or so) after the Big Bang. So the answer to Stephen’s question about whether ALL the light from a “smaller” early universe has been lost is basically no. Light from the earliest universe hasn’t escaped us — it is coming at us from all directions, from the far off and distant parts of space.

Some Notes about the Expansion of the Universe:

** It might help to think of this process of “expansion” as space itself stretching larger — something like the surface of a balloon. Imagine someone placed two star stickers close to each other on one side of an empty balloon, and a third star sticker on the other side of the balloon. If someone were to blow into the balloon, the stars would all be pulled apart — because the surface they were on was expanding.

As you know, the universe isn’t 2-dimensional like the surface of a balloon. How can we describe this in three dimensions? Here’s one analogy: Imagine someone has made raisin bread dough. He sets the dough out to rise. As the dough rises, the raisins in the dough move away from each other as the space between them increases.

What’s nice about both these analogies is that you can anticipate the idea that objects far from you travel faster away from you than objects that are closer. (Think about the motion of the stars on the balloon. The two stars next to each other don’t move away from each other as quickly as the star on the other side of the balloon.) This is something scientists have observed — those objects that are farthest away are the ones traveling the fastest relative to us.

Why is this important? If an object is moving quickly away, it means that any light sent from that object will be Doppler shifted. (One example of a Doppler shift is the changing pitch of an ambulance as it zooms past.) In the case of light, Doppler shifts stretches out the wavelength, turning high-energy gamma waves into microwaves. The high-energy radiation from the “surface of last scattering”, for example, has been stretched by the expansion of the universe to the microwave part of the spectrum. So in order to see things that are very far away -we need to look at the microwaves that arrive at Earth –not in the UV or visible light.

You can see some useful information about the electromagnetic spectrum here: https://imagine.gsfc.nasa.gov/science/index.html

If you’re interested in the challenges of measuring this light from long ago, you can search for information on “microwave astronomy” or check out all the resources on the NASA’s W-Map mission webpage, or even the future James Webb Space Telescope site. (The Webb telescope will be an orbital telescope (like the Hubble) that collects microwave light. There are also ground-based microwave telescopes.) Have fun!

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